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| United States Patent |
5,262,481
|
|
|
November 16, 1993
|
Moisture-cross-linkable thermoplastic adhesive compositions, uses
thereof, and modified components for use in said adhesive compositions
Abstract
Monocomponent thermoplastic adhesives that are spontaneously cross-linkable
by moisture and modified tackifier resins and other modified components
for use therein that are obtained by reacting the same with an excess of
polyisocyanate, and more specifically with MDI. The thermoplastic
adhesives formulated with these modified resins have good resistance to
creep at temperatures as high as 160.degree. C.
| Inventors:
|
Jammet Jean C. (Glisolles, FR)
|
| Assignee:
|
CECA, S.A. (FR)
|
| Appl. No.:
|
725049 |
| Filed:
|
July 3, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
525/125; 525/123; 525/127; 525/457 |
| Intern'l Class: |
C08L 031/04; C08L 075/08; C08L 091/06 |
| Field of Search: |
525/123,125,127,457
|
References Cited
U.S. Patent Documents
| 3463753 | Aug., 1969 | Gonzenbach et al. | 524/187.
|
| 3718712 | Feb., 1973 | Tushaw | 525/457.
|
| 4320211 | Mar., 1982 | Acharya et al. | 525/210.
|
| 4325853 | Apr., 1982 | Acharya et al. | 525/330.
|
| 4337297 | Jun., 1982 | Karim et al. | 525/125.
|
| 4377510 | Mar., 1983 | Ruckel et al. | 524/187.
|
| 4471106 | Sep., 1984 | Luecke et al. | 528/205.
|
| 4808255 | Feb., 1989 | Markevka et al. | 525/127.
|
| 4855382 | Aug., 1989 | Vanhaeren | 528/45.
|
| 4908408 | Mar., 1990 | Boutillier et al. | 525/124.
|
| Foreign Patent Documents |
| A10380379 | Jan., 1990 | EP.
| |
Other References
Chem Abstracts vol. 97, 1982.
Chem Abstracts vol. 107, 1987.
|
Primary Examiner: Seidleck; James J.
Assistant Examiner: Hamilton, III; Thomas
Claims
What is claimed is:
1. A monocomponent thermoplastic adhesive composition comprising:
from 5 to 80% by weight of a thermoplastic polymer,
from 0 to 60% by weight of a wax and/or paraffin, and
from 10 to 80% by weight of a tackifier resin,
wherein the tackifier resin is a modified resin resulting from the reaction
of a polyisocyanate with a resin selected from a rosin and its polyol
esters, a styrene resin modified by a phenol, or a phenol-terpene, and the
isocyanate index of which is between 1.5 and 20%.
2. A monocomponent thermoplastic adhesive composition consisting
essentially of:
from 5 to 80% by weight of a thermoplastic polymer,
from 0 to 60% by weight of wax and/or paraffin; and
from 10 to 80% by weight of a tackifier resin,
wherein the tackifier resin is a totally or partially modified resin
resulting from the reaction of a polyisocyanate with a resin selected from
a rosin and its polyol esters, a styrene resin modified by a phenol, or a
phenol-terpene, and the isocyanate index of the thermoplastic composition
is between 1 and 15%.
3. A monocomponent thermoplastic adhesive polymer comprising:
from 5 to 80% by weight of a thermoplastic polymer,
from 0 to 60% by weight of a modified wax and/or paraffin; and
from 10 to 80% by weight of a tackifier resin,
wherein the mixture of modified wax and/or paraffin and tackifier resin
contains a sufficient amount of a modified wax, paraffin and tackifier
that the content of isocyanate of said composition is between 1 and 15%,
said tackifier resin resulting from the reaction of a polyisocyanate with
a tackifier resin selected from a rosin, a rosin derivative, a styrene
resin modified by a phenol, or a phenol-terpene resin, characterized in
that its content of free isocyanate is between 1.5 and 20%.
4. The composition of claim 3, wherein said tackifier resin results from
the reaction of a polyisocyanate with a phenol-terpene type resin of the
diphenol-terpene type.
5. The composition of claim 3, wherein said polyisocyanate is
diphenylmethane diisocyanate.
6. The composition of claim 3, wherein said modified wax results from the
reaction of a polyisocyanate with a wax selected from a
polyethylene-glycol wax or an ethylene/acrylic acid copolymer wax.
Description
BACKGROUND OF THE INVENTION
The present invention pertains to new improved components for thermoplastic
adhesives intended to cross-link slowly after application.
Thermoplastic adhesives are compositions intended to assure the bond
between diverse materials. These adhesives are in the form of solid, stiff
compositions at room temperature, but they melt when the temperature is
raised and then develop adhesive properties in their melted state and
continue to adhere strongly to the supports on which they were deposited
upon returning to the solid state.
Generally, thermoplastic adhesives are compositions based on a
thermoplastic polymer, e.g., EVA (ethylene-vinyl acetate copolymer), APAO
(atactic poly .alpha.-olefin) and thermoplastic rubber, and which also
contain, depending on the case:
(i) tackifier resin, a substance which is defined by its own specific
physical and chemical properties, notably its compatibility in the hot
state with polymer bases, and by the thermomechanical properties that it
induces in the polymer mixtures in which it is contained, i.e., an
improvement in their wettability and fluidity, an increase in their glass
transition temperature T.sub.g and a reduction in their modulus;
(ii) plasticizers, substances that have the role of reducing the glass
transition temperature T.sub.g and of making the mixture more pliable;
(iii) waxes and/or paraffins, which act as simple charges in the solidified
material, which can accentuate the elastic performance and improve the
mechanical performance, and as solvents in the melted product, which
decrease the viscosity;
(iv) oils, which decrease the viscosity and improve the pliability at room
temperature; and/or
(v) mineral charges, fillers (chalk), or pigments (titanium oxide).
Thermoplastic adhesives are extensively employed in industry, e.g., in
packaging, furniture, labeling, binding and plumbing, where they are
appreciated for their moderate price, ease of use, quick setting, lack of
solvent and, generally, their low toxicity. Nevertheless, they have two
major defects. On the one hand, their mechanical resistance is low and, on
the other hand, their heat performance is very limited. These defects
restrict their use to nonstructural bonding, i.e., to the bonding of
materials that themselves have low mechanical resistance, such as paper,
thin plastic films, cardboard and non-woven materials. Their mediocre heat
performance is the corollary of their thermoplasticity, and originates in
their composition itself, based on thermoplastic polymers such as
ethylene/vinyl acetate copolymers, thermoplastic rubbers and atactic
propylene, and limits their use to a temperature range of circa
-30.degree. C. to +60.degree. C.
Bonding with a thermoplastic adhesive is carried out by depositing the hot
(circa 160.degree. C.) product which has low viscosity (viscosity between
circa 0.1 and 100 Pa.s) on the support, then almost immediate
counterbonding with the second support. Setting is effected by simple
cooling, by passage through a phase transition: crystallization and/or
glass transition. This system is perfectly reversible and if the adhesive
is reheated, it remelts. But even before remelting, it loses some of its
strength which is manifested by a tendency to creep, which can result in
the relative displacement of the bonded pieces or even their total
detachment. This drawback can even be manifested with certain very pliable
adhesives at room temperature.
For several years, chemists have explored many approaches in attempts to
resolve this defect. These approaches can be grouped into bicomponent
products and monocomponent products. The term "bicomponent" means a
formulation which is necessarily presented in the form of two products
that must be mixed together at the moment of use, because the mixture
itself is reactive and its properties develop as soon as the mixture is
created. As its name indicates, a "monocomponent" product is comprised of
a single product which is stable when stored, at least for a reasonable
period of time. The bicomponent product approach, notably based on
epoxides, is difficult and implementation is delicate. The monocomponent
approach is more fertile with monocomponents with high melting points or
monocomponents that can be cross-linked by heat, by the oxygen in the air,
by irradiation (UV, electron jet, etc.), by the moisture in the air or the
supports to be bonded.
Improvement in heat performance by selecting thermoplastic components with
high crystallization temperatures has given birth to thermoplastic
adhesives based on polyamides and polyesters. These products, which remain
expensive, have the drawback that they can only be applied at high
temperatures and therefore undergo noteworthy thermal degradation and, in
addition, the bonding quality is mediocre under ordinary application
conditions, probably because the crystallization of these products takes
places too quickly and is associated with poor wetting.
Chemical cross-linking can be created with heat, rather like the
cross-linking of rubber by means of vulcanization. The chemical process of
this process is extremely variable. A peroxide path is known with monomers
and/or copolymers with potentially reactive residual double bonds, e.g.,
acrylic copolymers, or the urethane-acrylates from the Basenden Company.
The product, which has a phase transition at circa 40.degree.-80.degree.
C., is applied at circa 60.degree.-100.degree. C. and cross-linked at high
temperature (circa 180.degree. C. for 10 minutes). Another approach that
has been followed is that of polycondensation of EVA-type polymers
modified by an hydroxyacrylate, which are cross-linked at circa
180.degree. C. for 10 minutes with a blocked isocyanate. Cf. the French
patent application published as No. FR 2,616,155 (ATOCHEM) and European
Patent Application No. 0 302 620 (EXXON CHEMICAL). These approaches are
restrictive. They impose a high-temperature curing step, contrary to most
applications. In addition, these products can begin the cross-linking
process as they are applied, which creates the risk of damaging the
application equipment.
The formulations based on chemical cross-linking using the oxygen from the
air use alkyl boranes. They are more experimental than industrial in
nature.
UV irradiation cross-linking was proposed by Dynamit Nobel with polyesters
that are sensitive to UV radiation, which can be applied around 50.degree.
C. SHELL employs the same principle with its KRATON 1320 X. This aproach
is only applicable to the cross-linking of thin layers of product and its
application is very specific (e.g., use for coating products).
Cross-linking using moisture has been exploited for many years by adhesives
based on polyurethane-polyols with a low melting point, i.e., a T.sub.m
between 30 and 50.degree. C. and/or with a high glass transition
temperature, i.e., a T.sub.g- 40.degree. C. and +35.degree. C. (cf.
notably European Patent No. 0 107 097, FULLER). This approach is also
implemented with silicone mastics and thermoplastic adhesives based on EVA
grated with a silanol (cf., notably British Patent No. 2 197 326, SWIFT).
For these latter products, the hot stability is very mediocre and there is
a very intense increase in the viscosity when the product remains exposed
for more than three hours at circa 150.degree. C. This is obviously a
major constraint in the manufacture and use of thermoplastic adhesives.
There have also been proposed adhesives based on EVA/liquid polyurethane
prepolymers/tackifier resin mixtures in which the PPU prepolymer (the
abbreviated term "PPU prepolymer" is used to designate the polyurethane
prepolymers) is a conventional elastomer base that is almost liquid at
room temperature and has excess free --NCO groups that make it
cross-linkable with water. The PPU prepolymers are obtained in a known
manner by reaction of an excess of polyisocyanate monomer with polyol
monomers.
Studies have shown that in order to obtain proper cross-linking, the level
of PPU prepolymer in the formulation must reach circa 30% by weight. The
product is then highly softened and its cohesion at room temperature is
considerably below that of a conventional thermoplastic adhesive as long
as cross-linking has not been performed, which requires circa 24 hours.
SUMMARY OF THE INVENTION
It has now been discovered that it is possible to endow certain
conventional bases in the formulations of thermoplastic adhesives with the
property of reacting with moisture, i.e., the tackifier resins, but also
certain waxes, oils and plasticizers, by reaction with bifunctional or
multifunctional isocyanates, and to exploit these modified components for
the formulation of monocomponent thermoplastic adhesives that are
post-cross-linkable by moisture and useful for the creation of adhesives
that resist creep at temperatures as high as 160.degree. C.
Thus, the present invention is comprised of modified tackifier resins for
the formulation of monocomponent thermoplastic adhesives with excellent
creep resistance, modified resins which result from the reaction of
polyisocyanates with tackifier resins, and the free isocyanate level of
which is between 1.5 and 20%, preferentially between 5 and 15%. The
invention also comprises other conventional components of thermoplastic
adhesives such as modified waxes which have been provided with the quality
of reactivity with moisture by controlled reaction with polyisocyanates.
DETAILED DESCRIPTION
The modifiable tackifier resins that are useful for the invention belong to
the group constituted by phenol-terpenes, styrenes (styrene,
.alpha.-methylstyrene) modified by phenol, rosins, and their derivatives,
notably their polyol esters. All of these substances have --OH groups, as
part of alcohol or carboxylic functions, capable of reacting with
isocyanates. Any mixture of these resins with polyether polyols and/or
polyester polyols can also result in a post-cross-linkable product. French
Patents No. 2 313 817 and No. 2 307 836 (Resin and Terpene Derivatives)
provide a particularly valuable description of resins of the diphenlic
terpene type that are particularly useful for the invention and their
disclosure is incorporated herein by reference.
The modifiable waxes that are useful for the invention belong to the
groups:
(i) of polyethylene-glycols,
(ii) of ethylene/acrylic acid copolymers, and
(iii) of ethylene/maleic anhydride copolymers.
The polyisocyanates that are employed to create the modified tackifier
resins, and the modified waxes, plasticizers, such as castor oil, and
polymers in accordance with the invention, are the aliphatic or aromatic
polyisocyanates. The diisocyanates, particularly diisocyanate
diphenylmethane (MDI), are preferred.
All of these modified components in accordance with the invention, which
can be prepared according to a process than can be perfectly controlled
and which guarantees their quality, can be employed as components in the
manufacture of cross-linkable thermoplastic adhesives. These
cross-linkable thermoplastic adhesives, which are also part of the
invention, are generally comprised of 5 to 80% of a thermoplastic polymer,
from 0 to 60% of waxes and/or paraffins and from 10 to 80% of tackifier
resins (these percentages are the limits within which each of the
components can be employed, in relation to a final composition the sum of
the components of which is, of course, set at 100%). The polymer bases
employed are those that are usually selected by the expert in the field of
formulations containing a tackifier resin, i.e., bases whose viscosity in
the melted state is between 0.1 and 200 Pa.s. The tackifier resins are the
modified resins as described above, or mixtures of ordinary and modified
resins. In the place of waxes and plasticizers ordinarily used, use can be
made of modified waxes and plasticizers of mixtures of these ordinary
components with modified components. In all cases, the adhesive's overall
content of modified components must be such that the isocyanate level of
the formulated adhesive is between circa 1 and 15, including the
contribution that can be provided to the formulation by the possible
presence of a PPU prepolymer. The formulation and packaging methods for
the manufacture of ordinary thermoplastic adhesives, which are well known
by the expert in the field, are applied to these components. It is obvious
that all precautions must be taken to avoid contact between these reactive
components and moisture, especially during their storage and during their
use for the manufacture of the adhesives in which they are incorporated.
When using them, the cross-linkable thermoplastic adhesives in accordance
with the invention can be treated like conventional thermoplastic
adhesives while taking the necessary precautions to keep them away from
ambient moisture until they have been definitively applied. Specifically,
they can be applied industrially by means of special equipment that was
previously developed by companies such as NORDSON and MELTEX for
thermoplastic adhesives that are cross-linkable by PPU polymers.
Thus, the invention provides the expert in the field with a new technical
means:
(i) for creating thermoplastic adhesives with better creep performance, on
the basis of conventional formulations;
(ii) for selectively varying their characateristics, and notably
(iii) for creating products with very short setting times, based on the
very rapid crystallization of the ethylene in EVA and ethylene-acrylates,
on the glass transition temperature of styrene (T.sub.g= 90.degree. C.)
for the case of functionalized thermoplastic rubbers;
(iv) for formulating thermoplastic adhesives with long open times with
functionalized thermoplastic rubber bases as well as with the bases cited
above in which the PPU prepolymer initially plays the role of a
plasticizer;
(v) for improving pliability by means of cross-linkable polymer bases; and
(vi) for improving the initial cohesion of the product by means of reducing
the amount of PPU prepolymer and compensation for this reduction by means
of the cross-linkable tackifier resin.
The well known defects of the cross-linkable thermoplastic adhesives of the
prior art have been eliminated from the thermoplastic adhesives in
accordance with the invention. For example, they have a much lower
sensitivity to overheating than silanated EVA, better setting time, a
longer open time and better cohesion than the thermoplastic adhesives
constituted of PPU prepolymers and better cohesion than the
EVA/PPU/tackifier resin systems.
Among the valuable and useful applications of the thermoplastic adhesives
in accordance with the invention can be cited the manufacture of
scrubber-sponges. These objects are usually manufactured by bonding an
abrasive pad onto a cellulose sponge block using a thermpolastic adhesive.
It is easy to see all of the advantages than can be achieved by employing
a thermoplastic adhesive in accordance with the invention which bestows on
the objects excellent resistance to very hot water even to the
temperatures of the dishwasher into which these objects can accidentally
slip. Another valuable application is the manufacture of sterilizable foam
mattresses in which can be provided, by means of this invention,
thermoplastic adhesives with the long open time required by the spraying
operation and the counter-bonding of foam after a certain delay, with the
unit being made resistant to sterilization by its spontaneous
post-cross-linking. We can also cite the bonding of joints for carpentry
required to be exposed to high temperatures., e.g., transport containers
or storage room, and the bonding of packaging that must be microwave
resistant.
The invention will be further described in connection with the following
examples which are set forth for purposes of illustration only.
EXAMPLES
In the following examples, reference is made to open time measurements and
to the SAFT test.
Measurement of the open time is performed by depositing on a cardboard
support a circa 1.5-wide line of thermoplastic adhesive at circa
170.degree. C. It is allowed to cool to room temperature until there is
seen good bonding on a second support of counter-bonded cardboard. The
maximum waiting time required to achieve good bonding (ripping of the
cardboard) is the open time.
To carry out the SAFT test, bonding is performed on a cardboard "shearing"
test specimen, 2.5 cm.times.2.5 cm. For this, one applies at
.apprxeq.130.degree. two threads of adhesive, circa 1 mm in diameter and
circa 2.5 cm apart, at one end of the cardboard strip. A second strip of
cardboard is counter-bonded in an overhanging position. The setting time
of the products is several seconds (circa 10 seconds). In order to obtain
ripping of the cardboard, a force of 0.5 daN in applied in the plane of
the unit. In practice, the unit is suspended vertically, a weight of 500g
is attached at the end of the test specimen and the unit is subjected to a
5.degree. C. increase in temperature every 30 minutes, starting at
40.degree. C. The SAFT performance is the temperature at which the bond
yields.
The immediate creep temperature is the temperature obtained for a bond that
has been aged for less than two hours before the test. The creep
temperature can also be measured on test specimens that have been allowed
to age for several days at room temperature.
The following tackifier resins were employed in the examples below:
(i) Uratack 68520 (DSM) which is an .alpha.-methyl styrene resin modified
with phenol; its hydroxyl index I.sub.OH= 36; or
(ii) resins 8923, 8924, 8941 (DRT), which are phenol-terpene type resins;
their hydroxyl indices are 100, 120 and 129, respectively.
An important characateristic of the modified resins in accordance with the
invention is their content of free isocyanate. This content is expressed
by weight in grams of free NCO per 100 grams of modified resin and it is
determined with the standard AFNOR 52132.
EXAMPLE 1
(a) Synthesis of cross-linkable resin R1
In order to obtain the desired reaction and a final % of NCO of 10%, the
following mixture was held at 100.degree. C. for 4 hours:
______________________________________
* Uratack resin 68520 185 g
* MDI 100 g
* Triethylamine 0.2%
______________________________________
(The triethylamine acts as a catalyst of the reaction of --OH with the
isocyanate).
(b) Formulation of the thermoplastic adhesive
The adhesive was prepared by intimately mixing the following ingredients at
130.degree. C., and continuing for circa 2 hours.
______________________________________
EVA 40/55 10.5% by weight
EVA 45/7.5 9.5%
Resin R1 50%
PPU prepolymer(XPU 6778Al)
30%
______________________________________
in which EVA 40/55 is an ethylene/vinyl acetate copolymer with 40% vinyl
acetate; Melt Index=55; in which EVA 45/7.5 is an ethylene/vinyl acetate
copolymer with vinyl acetate; Melt Index=7.5; and in which the PPU
prepolymer XPU 6778Al is a prepolymer from CECA S.A., formed from MDI and
with an NCO content of 7.
The adhesive formulated in this manner had a final NCO content of 7.1%.
(c) Evaluation of the performance of the thermoplastic adhesive
Using the SAFT test, a rupture temperature of 60.degree. C. was measured on
the shearing test specimen 2 hours after application of the adhesive. The
result was greater than 160.degree. C. for a shearing test specimen 5 days
after application of the adhesive.
EXAMPLE 2
A thermoplastic formulation was prepared under the same conditions as in
Example 1, but with the following composition:
______________________________________
Resin R1 40%
EVA 33/400
35%
EVA 28/800
25%
______________________________________
in which EVA 33/400 is an ethylene/vinyl acetate copolymer with 33% vinyl
acetate; Melt Index=400; in which EVA 28/800 is an ethylene/vinyl acetate
copolymer with 28% vinyl acetate; Melt Index=800.
The adhesive formulated in this manner had a final NCO content of 4%. The
cross-linking time required for this formulation is long. The immediate
SAFT performance (after 2 hours) was 65.degree. C.; the SAFT performance
after 15 days of aging in ambient air was 115.degree. C.
EXAMPLE 3
Modification of phenol-terpene based tackifier resins 8923 and 8924 (DRT).
(a) Synthesis of cross-linkable resin R2
In order to obtain the desired reaction, with a final isocyanate content of
7%, a mixture of the following was held at .apprxeq.90.degree. C. for
circa 4 hours.
______________________________________
DRT 8923 100 g
DRT 8924 41 g
MDI 80 g
DBTL (dibutyl tin dilurate),
0.3%
catalyst
______________________________________
(b) Formulation of the thermoplastic adhesive
The adhesive was prepared by mixing the following ingredients at
.apprxeq.130.degree. C. for 2 hours.
______________________________________
EVA 51/18 10%
EVA 40/55 20%
Resin R2 20%
Kristallex F100 30%
Prepolymer NM36 20%
______________________________________
in which EVA 51/18 is an ethylene/vinyl acetate copolymer with 51% vinyl
acetate; Melt Index=18, (Vynathene EY 905 from U.S.I.); in which EVA 40/55
is an EVA copolymer with 40% vinyl acetate; Melt Index= 55; Elvax 40 (du
Pont); in which prepolymer NM 36 is a conventional liquid PPU prepolymer
from CECA S.A., resulting from the condensation of polyols and MDI; final
NCO index 32 7; and in which Kristallex F100 (Hercules) is an
.alpha.-methyl styrene resin.
The formulated adhesive had a final NCO of 2.8%.
(c) Evaluation of the performance of the thermoplastic adhesive
______________________________________
Immediate SAFT performance
55.degree. C.
SAFT performance after 2 days
>160.degree. C.
Open time 20 seconds
______________________________________
EXAMPLE 4
Use is made of a modified tackifier resin of the phenol-terpene type (R3).
(a) Modification of the resin
Resin 8941 (DRT) is condensed with MDI so as to obtain a modified resin
(R3) whose final free isocyanate content is 10%.
(b) Formulation of the thermoplastic adhesive
The following were mixed at 130.degree. C. for 1.5 hours:
______________________________________
EVA 33/400 30%
EVA 28/800 10%
Kristallex F100 10%
Rosin resin ester
15%
Resin R3 30%
______________________________________
(c) Evaluation of the thermoplastic adhesive
______________________________________
Measured NCO content of the
3.15%
thermpolastic adhesive
Viscosity at 140.degree. C.
16 Pa .multidot. s
Immediate SAFT performance
65.degree. C.
SAFT performance after >160.degree. C.
two days:
______________________________________
While the invention has been described in connection with a preferred
embodiment, it is not intended to limit the scope of the invention to the
particular form set forth, but on the contrary, it is intended to cover
such alternatives, modifications, and equivalents as may be included
within the spirit and scope of the invention as defined by the appended
claims.
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